✅Answer: Please pay attention to the following narrations:

1 قال امیرالمؤمنین في وصيته إلي ابنه الحسن :فَأَلْحِحْ عَلَیْهِ فی الْمَسْأَلَةِ یَفْتَحْ لَکَ أَبْوَابَ الرَّحْمَةِ لَا یُقَنِّطُکَ إِنْ أَبْطَأَتْ عَلَیْکَ الْإِجَابَةُ فَإِنَّ الْعَطِیَّةَ عَلَی قَدْرِ الْمَسْأَلَةِ و رُبَّمَا أُخِّرَتْ عَنْکَ الْإِجَابَةُ لِیَکُونَ أَطْوَلَ لِلْمَسْأَلَةِ و أَجْزَلَ لِلْعَطِیَّةِ رُبَّمَا سَأَلْتَ الشَّیْءَ فَلَمْ تُؤْتَاهُ و أُوتِیتَ خَیْراً مِنْهُ عَاجِلًا أَوْ آجِلًا أَوْ صِرْتَ إِلَی مَا هُوَ خَیْرٌ لَکَ فَلَرُبَّ أَمْرٍ قَدْ طَلَبْتَهُ و فِیهِ هَلَاکُ دِینِکَ وَ دُنْیَاکَ لَوْ أُوتِیتَهُ

📚Bihar, Vol. 93, p. 301

The Commander of the Faithful (peace be upon him) said in his will to Imam Mujtaba (peace be upon him): 'Be persistent in supplication and asking of Allah so that He may open the doors of mercy for you. If your response is delayed, do not despair, for the gift is in proportion to the request, and sometimes your response is delayed so that the request is prolonged (and consequently) the gift and grant is greater!'

'It may be that you ask for something and it is not given to you, and something better than it is given to you in the short or long term, or you go after something that is better for you, for it may be that there is a matter that you have sought, while if it is given to you, it would lead to the destruction of your religion and your world!'

2 عن أبي عبداللَّه في حديث قال: مَنْ قَامَ مِنْ آخِرِ اللَّيْلِ فَذَكَرَ اللَّهَ تَنَاثَرَتْ عَنْهُ خَطَايَاهُ فَإِنْ قَامَ مِنْ آخِرِ اللَّيْلِ فَتَطَهَّرَ و صَلَّى رَكْعَتَيْنِ و حَمِدَ اللَّهَ و أَثْنَى عَلَيْهِ و صَلَّى عَلَى النَّبِيِّ لَمْ يَسْأَلِ اللَّهَ شَيْئاً إِلَّا أَعْطَاهُ إِمَّا أَنْ يُعْطِيَهُ الَّذِي يَسْأَلُهُ بِعَيْنِهِ و إِمَّا أَنْ يَدَّخِرَ لَهُ ما هُوَ خَيْرٌ لَهُ مِنْهُ

📚Al-Kafi, Vol. 3, p. 468

Imam Sadiq (peace be upon him) said: 'Whoever rises at the end of the night and remembers Allah, his sins will fall away from him. And if he rises at the end of the night and performs ablution and prays two rak'ahs and praises Allah and glorifies Him and sends blessings upon the Prophet (peace be upon him and his family), he does not ask Allah for anything except that He grants it to him: either He grants him the very thing he asked for, or He stores for him what is better than it!'

3 قال الصادق علیه السلام: الْمُؤْمِنُ مِنْ دُعَائِهِ علَی ثَلَاثٍ إِمَّا أَنْ یُدَّخَرَ لَهُ و إِمَّا أَنْ یُعَجَّلَ لَهُ و إِمَّا أَنْ یُدْفَعَ عَنْهُ بَلَاءٌ یُرِیدُ أَنْ یُصِیبَهُ

📚Bihar, Vol. 75, p. 137

Imam Sadiq (peace be upon him) said: 'The believer in his supplication is in one of these three things: either it is stored for him, or it is given to him quickly, or a tribulation that was about to reach him is averted from him!'

4 إسحاق بن عمار عن أَبِي عبداللَّه قال: إِنَّ الْمُؤْمِنَ لَيَدْعُو اللَّهَ عَزَّوَجَلَّ في حَاجَتِهِ فَيَقُولُ اللَّهُ عَزَّوَجَلَّ أَخِّرُوا إِجَابَتَهُ شَوْقاً إِلَى صَوْتِهِ و دُعَائِهِ فَإِذَا كَانَ يَوْمُ الْقِيَامَةِ قَالَ اللَّهُ عَزَّوَجَلَّ عَبْدِي دَعَوْتَنِي فَأَخَّرْتُ إِجَابَتَكَ و ثَوَابُكَ كَذَا و كَذَا و دَعَوْتَنِي في كَذَا و كَذَا فَأَخَّرْتُ إِجَابَتَكَ و ثَوَابُكَ كَذَا و كَذَا قال فَيَتَمَنَّى الْمُؤْمِنُ أَنَّهُ لَمْ يُسْتَجَبْ لَهُ دَعْوَةٌ فِي الدُّنْيَا مِمَّا يَرَى مِنْ حُسْنِ الثَّوَاب

📚Al-Kafi, Vol. 2, p. 490

Imam Sadiq (peace be upon him) said: 'Indeed, the believer supplicates to Allah Almighty for a need, and Allah Almighty, due to His longing for his voice and supplication, says: 'Delay his response.' And when the Day of Resurrection comes, Allah Almighty says: 'O My servant! You called Me and I delayed your response, and your reward is such and such, and you supplicated for such and such, and I delayed your response, and your reward is such and such!'

The Imam said: 'So the believer, because of the good reward he sees, wishes that none of his supplications had been fulfilled in the world.'

✅Fulfillment (Istijabah) in the lexicon means to answer, accept, and grant. According to the above and other narrations, the fulfillment of a supplication means its acceptance, as a result of which either the very need requested is given to the person, or something better than it is given to him, or because of that supplication, a tribulation is averted from the person, or it is stored for his hereafter, and such a reward is given to the person on the Day of Resurrection that he wishes that none of his supplications had been fulfilled in the world!

Understanding the meaning of the fulfillment of supplication removes despair and hopelessness from us, for the person becomes aware that the fulfillment of a supplication is not only the granting of the very need itself.