Musnad Ahmad 529:12

Haywah ibn Shurayh narrated to us, Baqiyyah narrated to us, Buhayr ibn Sa’d narrated to me, from Khalid ibn Ma’dan, from ‘Utbah ibn ‘Abd, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If a man were to fall upon his face from the day he was born until the day he dies of old age, in the pleasure of Allah, he would consider it insignificant on the Day of Resurrection."