The Messenger of Allah ﷺ said, "Do not race ahead of me in bowing or prostrating, for whatever I gain over you when I bow, you will catch up with me when I rise. I have grown heavy."
قال رسول الله ﷺ: لا تبادروني بركوع ولا بسجود، فإنه مهما أسبقكم به إذا ركعت تدركوني به إذا رفعت، إني قد بدنت.
Abu Dawud (619) and Ibn Majah (963) both narrated it through the route of Yahya bin Saeed, from Ibn Ajlan, who said, Muhammad bin Yahya bin Habban narrated to me, from Ibn Muhayriz, from Muawiya bin Abu Sufyan, who mentioned it. Its isnad is hasan, for Muhammad bin Ajlan's hadith is hasan. Ibn Khuzayma (1594) and Ibn Hibban (2230) also recorded it, both through the route of Ibn Ajlan, in similar fashion, except that Ibn Khuzayma made Yahya bin Saeed a corroborating narrator alongside Ibn Ajlan in one of his chains. With this corroboration, the hadith rises to the level of sahih.His saying, "catch up with me in it when I raise my head," means that it does not harm you that I raise my head, since something of it still remains for you to catch up on while I am standing, before I prostrate. The Prophet ﷺ, when he raised his head from ruku, would say a lengthy supplication.As for his saying, "indeed I have become badantu," it is transmitted in two ways. One is badduntu, with the dal doubled, meaning advanced in age. It is said, "the man badana," meaning he grew old. The other is baduntu, with the dal having a damma and not doubled, meaning an increase in bodily bulk, that is, carrying more flesh. Ayesha reported that the Messenger of Allah ﷺ, when he grew advanced in years, put on more flesh in his body. Both advancement in age and increase of flesh make the body heavier and slow it down in movement. So said Khattabi.