Musannaf Ibn Abi Shayba 3:182

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from Sulayman ibn Suhaym, from Ibrahim ibn ‘Ubayd Allah ibn Ma‘bad, from his father, from Ibn ‘Abbas, who said: The Prophet (peace and blessings of Allah be upon him) said: "As for the bowing (ruku‘), then glorify the Lord in it. And as for the prostration (sujud), then strive in supplication, for it is worthy that it be answered for you."