Musnad Ahmad 25:1529

Yahya ibn Abd al-Malik ibn Abi Ghaniyah narrated to us, he said: Al-Awwam ibn Hawshab informed us, he said: I asked Mujahid about the prostration that is in [Surah] Sad. He said: Yes, I asked Ibn Abbas about it, and he said: Do you read this verse: {And among his descendants, David and Solomon} and at its end: {So by their guidance, take an example} [Al-An'am: 90]? He said: "Your Prophet, may Allah bless him and grant him peace, was commanded to take an example from David."