This is an authentic mursal chain of transmission. It has also been reported in a connected, abbreviated form through several routes from Humayd al-Tawil, who said: Bakr narrated to me that Abu Sa'id al-Khudri saw in a dream that he was writing Surah Sad. When he reached its verse of prostration, he saw the inkwell, the pen, and everything before him fall into prostration. He said: He related it to the Prophet, peace and blessings be upon him, who thereafter continued to prostrate upon reciting it.

Ahmad recorded it (3/78 and 84) through Yazid ibn Zuray' and Ibn Abi 'Adi, and al-Hakim (2/432) through Hammad ibn Salamah; all three narrated it from Humayd with this chain. Al-Hakim made no comment on it, while al-Dhahabi said: “It meets Muslim’s criteria.” I say: It is as he said; indeed, it meets the criteria of both al-Bukhari and Muslim, were it not for its apparent mursal nature, owing to the wording, “that Abu Sa'id...” This is supported by Hushaym’s narration: “Humayd al-Tawil informed us, from Bakr ibn Abdullah, who said: A narrator informed me from Abu Sa'id,” and then related it. However, he said: “So I returned to the Messenger of Allah, peace and blessings be upon him, and informed him, and he instructed that one prostrate upon reciting it.” Al-Bayhaqi recorded it (2/320).

It may, however, be said that this narration is anomalous, because it contradicts the narration of the three reliable narrators. Yet this narration itself is not connected, as we have mentioned. And Allah knows best.

As for the corroborating report, its supplication reads: “O Allah, record for me through it a reward with You; make it a treasure stored for me with You; remove from me through it a sin; and accept it from me as You accepted it from Your servant David.” It was recorded by al-Tirmidhi (579 and 3420), Ibn Majah (1/325), Ibn Hibban (691) through Ibn Khuzaymah, al-Hakim (1/219), and al-Bayhaqi (2/320)