Answer:

The evidence for the obligation, importance, and necessity of "awaiting the reappearance" is abundant. In this brief response, a few points are highlighted:

1⃣ Imam Sadiq (peace be upon him) said in a hadith:

«... مَنْ سَرَّهُ أَنْ يَكُونَ مِنْ اَصْحابِ اْلقائِم عليه السلام فَلْيَنْتَظِرْ وَلْيَعْمَلْ بِالْوَرَعِ وَمَحاسِنِ اْلاَخْلاقِ، وَهُوَ مُنْتَظِرٌ. فَإِنْ ماتَ وَقامَ اْلقائِمُ بَعْدَهُ، كانَ لَهُ منَ اْلاَجْر مثْلُ اَجْر مَنْ اَدْرَكَهُ، فَجدُّوا وَانْتَظِروُا هَنيئاً لَكُمْ أَيّتُهَا اْلعصابَةُ اْلمَرْحُومَةُ»

"Anyone who wishes to be among the companions of Imam al-Mahdi (may Allah hasten his reappearance) must be patient, practice piety and good morals, and, at the same time, be in a state of anticipation. If he dies, and Imam al-Mahdi appears after him, the reward for understanding him will be granted to him. So, strive and await! May the blessings of these virtues be upon you."

📚 Ghaybat-e-Nu'mani, p. 200

2⃣ Imam Sadiq (peace be upon him) addressed his companions, saying:

«اَلا اُخْبِركُمْ بِما لايَقْبَلُ اللَّه عَزّوَجلّ، مِنَ اْلِعبادِ عَمَلاً إِلاّ بِهِ؟ فَقُلْتُ: بلى؛

فَقالَ: شَهادَةُ أَنْ لا اِلهَ اِلاّ اللَّه، وَأَنّ مُحَمَّداً عَبْدُهُ وَرَسُولُهُ، وَاْلاِقْرارُ بِما اَمَرَ اللَّه، وَاْلوِلايَةُ لَنا، وَالْبَرائَةُ مِنْ اَعْدائِنا - يَعنِى اْلاَئِمَّة خاصَّةً - وَالتَّسْلِيمُ لَهُمْ، وَاْلوَرَعُ، وَاْلاِجْتِهاد وَالطُّمَأْنِينَةُ، وَاْلاِنْتِظارُ لِلْقائِمِ عليه السلام... »

"... Should I not inform you of something that Allah, the Almighty, accepts no deed from His servants except through it?

I said, 'Certainly.'

He said: 'Bearing witness to the oneness of Allah, acknowledging that Muhammad (peace be upon him and his progeny) is His servant and messenger, confirming what Allah has commanded, the wilayah (guardianship) of us, the Ahl al-Bayt, and enmity towards our enemies, obedience to their commands, piety, effort (striving in deeds and worship), patience, and anticipation for the coming of al-Qa'im (Imam al-Mahdi)...'"

✅ As seen, anticipating the appearance of Imam al-Mahdi is mentioned as one of the prerequisites for deeds to be accepted.

3⃣ Sheikh Saduq narrates through his chain from Abd al-Azim Hasani that he asked Imam al-Jawad (peace be upon him) about Imam al-Mahdi, whether he is the Mahdi or someone else.

Imam al-Jawad began speaking on his own, saying:

«يا اَبا اْلقاسِم! اِنّ اْلقائمَ عليه السلام مِنّا هُوَ اْلمَهْديّ الَذِّي يَجِب أَنْ يَنْتَظِرَ فِي غَيبَتِهِ، وَيُطاع فِي ظُهُورِهِ، وَهُوَ الثّالث مِنْ وُلْدِي... »

"O Abu al-Qasim! Indeed, our Qa'im (Imam al-Mahdi) is the same Mahdi; the one whose anticipation during his occultation and obedience when he reappears are obligatory. He is the third (generation) from my progeny..."

4⃣ Imam al-Jawad (peace be upon him) said: "

... اَفضَلُ اَعْمالِ شيعَتنا اِنْتِظار اْلفَرج...

The best actions of our Shi'a are awaiting the reappearance."

📚 Sheikh Saduq, Al-Kamal, vol. 2, p. 377

5⃣ Imam Sadiq (peace be upon him) is quoted in another narration regarding the virtue of those awaiting the reappearance:

«... يا اَبابَصير! طُوبى لِشيعَةِ قائِمِناعليه السلام اَلمُنتظِرين لِظهُورِهِ فِي غَيبَتِهِ، وَاْلمُطيعِينَ لَهُ فِي ظُهوُرِهِ، اُولئِك اُولِياءُ اللَّهِ اَلذّين لاخَوْفٌ عَلَيْهِمْ وَلاهُمْ يَحْزَنُون»؛

"O Abu al-Basir! Blessed are the Shi'a of our Qa'im, those who anticipate him during his occultation, and when he appears, they are obedient to him. They are the friends of Allah, who neither fear nor grieve."

📚 Kamal al-Din, vol. 2, p. 357

6⃣ "Sheikh Saduq narrates from Imam Sadegh (peace be upon him) that after introducing his son Imam Musa ibn Ja'far (peace be upon him) and mentioning that Imam Mahdi (peace be upon him), who is the completer of the twelve Imams, is from his progeny, he said:"

«... اَلمُنْتَظِر لِلثّانِى عَشَر مِنْهُمْ كَالشّاهِرِ سَيْفَهُ بَيْنَ يَدَيْ رَسوُلِ اللَّهِ صلى الله عليه وآله يَذُبُّ عَنْهُ...»

"Anyone who awaits the twelfth of them is like someone who unsheathes his sword and defends the Prophet of God (a.s) and his family, in his presence."

📚 Kamal al-Din, vol. 2, p. 335

✅ Important Note: Reading books like "Ghaybat-e-Nu'mani" and "Al-Kamal" by Sheikh Saduq is strongly recommended for acquiring knowledge about Imam Mahdi (may Allah hasten his reappearance), as not being aware of certain information about him may pose significant risks to our faith.