Narrated Mujahid:

I asked Ibn `Abbas, "Should we perform a prostration on reciting Surat-Sa`d?" He recited (the Sura) including: 'And among his progeny, David, Solomon..(up to)...so follow their guidance (6.84-91) And then he said, "Your Prophet is amongst those people who have been ordered to follow them (i.e. the preceding apostles).

Narrated Al-Awwam:

I asked Mujahid regarding the prostration in Surat Sa`d. He said, "I asked Ibn `Abbas, 'What evidence makes you prostrate?' He said, "Don't you recite:--'And among his progeny, David and Solomon..(6.84). Those are they whom Allah had guided. So follow their guidance.' (6.90) So David was the one of those prophets whom Prophet (Muhammad) was ordered to follow. David prostrated, so Allah's Messenger (Muhammad) performed this prostration too.'