Sayyiduna Utbah bin Abd (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "If a man were to fall on his face (in prostration) from the day he was born until the day he dies, so as to attain the pleasure of Allah Almighty through it, on the Day of Resurrection that man would consider even that insignificant."
Ahmad recorded it, and its chain is jayyid (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الزهد / 17680