There are two kinds of reports on asbab al-nuzul:

In the first kind (definite) the narrator clearly indicates that the event he relates is the sabab al-nuzul.

Example:

Narrated Ibn 'Abbas: the verse 'Obey Allah and obey the apostle and those of you (Muslims) who are in authority ...' (4: 59) was revealed in connection with 'Abdullah bin Hudafa bin Qais bin 'Adi when the Prophet appointed him as the commander of a sariyya (army detachment). [Bukhari, VI, No. 108.]

In the second kind (probable) the narrator does not indicate clearly that the event narrated is the sabab al-nuzul, but suggests this probability.

Example:

Narrated 'Urwa: Az-Zubair quarrelled with a man from the Ansar because of a natural mountainous stream at Al-Harra. The Prophet said: O Zubair, irrigate (your land) and then let the water flow to your neighbour. The Ansar said: O Allah's apostle (this is because) he is your cousin? At that the Prophet's face became red (with anger) and he said: O Zubair. Irrigate (your land) and then withhold the water till it fills the land up to the walls and then let if flow to your neighbour. So the Prophet enabled Az-Zubair to take his full right after the Ansari provoked his anger.

The Prophet had previously given an order that was in favour of both of them. Az-Zubair said: 'I don't think but this verse was revealed in this connection: But no, by your Lord, they can have no faith, until they make you judge in all disputes between them' (4: 65). [Bukhari, VI, No. 109.]