Our Shaykh, is this hadith authentic:
On the authority of Zaid bin Aslam Al-Adawi, may Allaah be pleased with him, from his father, may Allaah be pleased with him, the Messenger of Allaah, the Most High, sallallaahu 'alayhi wa sallam, said, “Jihad will remain sweet and green as long as rain continues to drop from the sky, yet there will come upon the people a time when their scholars will say, 'This is not the time for jihad.' So whoever witnesses this time, then let him know that this is the best time for jihad.” The Companions asked, ”O Messenger of Allaah, will anyone really say that?” He, sallallaahu 'alayhi wa sallam, replied, “Yes, he upon whom is the curse of Allaah, His Angels, and all of mankind.”
Ibn Asakir transmitted it. (Mashari-ul-Ashwaaq by Ibn Nuhaas, Vol 1,P.110)

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

The narration that you mentioned was narrated by Abu ‘Amr ad-Daani Al-Andalusi, the known Quran reciter, in his book: As-Sunan Al-Waaridah fi Al-Fitan wa Ghawaa’iliha was-Saa’ah wa Ashraatiha, from ‘Abdur-Rahmaan ibn Zayd ibn Aslam from his father Zayd ibn Aslam, who was a Taabi’i, so he did not meet the Prophet . Thus, his narration from the Prophet is considered a mursal hadeeth (i.e. a report narrated on the authority of some of the Taabi'oon [the generation that followed the Companions]), which is one of the categories of weak ahaadeeth.

Also, his son, Abdur-Rahmaan is classified as weak by many scholars.

Ibn Hajar quoted the statements of the scholars of hadeeth in classifying him as weak  in Tahtheeb at-Tahtheeb:

“Ad-Doori said that Ibn Ma’een said, 'His hadeeth is of no value.' Al-Bukhari and Abu Haatim said, 'Ali ibn Al-Madeeni classified him as very weak,' and Abu Daawood said, 'The children of Zayd ibn Aslam are all weak, and the best of them was ‘Abdullaah...' As-Saaji said, 'His hadeeth is rejected.' At-Tahaawi said, 'His hadeeth according to the scholars of hadeeth is at the extreme end of weakness.' Al-Harbi said, 'Others are more trustworthy than him.' Al-Jawzajaani said, 'The children of Zayd are weak.' Al-Haakim and Abu Nu’aym said, 'He narrated  fabricated ahaadeeth from his father.' Ibn al-Jawzi said, 'They (the scholars) agreed in a consensus that he is weak.'”

There is another narration with this meaning narrated by Ibn ‘Asaakir in Taareekh Dimashq (the History of Damascus) with a different wording on the authority of Anas in which the Prophet said, “Jihaad will remain sweet as long as rain continues to fall from the sky and as long as the earth brings out vegetation; and there will be a people from the east who will say there is no jihaad and no ribaat (i.e. living on the Muslims' front-line to protect the Muslims); those are the fuel of Hellfire. Rather, one day in the Cause of Allaah is better than emancipating one thousand slaves and better than the charity of all the people of the earth.”

Also, this hadeeth was cited by As-Suyooti in Al-Jaami’ Al-Kabeer, and he said, “It was reported by Ibn Asaakir from Anas, but he classified it as weak. The author of Kanz al-‘Ummaal mentioned it in Sunan al-Aqwaal wal-Af’aal.”

Allaah knows best.