This famous verse has been revealed in praise of the action of Imám `Alí (A).
Abu Dhar recounts, "Once, I was saying my prayers in the company of the Holy Prophet (S) when a beggar came to the mosque asking for some alms. Nobody gave him anything. `Alí (A) was in the state of Ruku` and he pointed out his ring to the beggar, who approached him and removed the ring from his finger." At this time the following verse was revealed:
Verily your guardian is Alláh and His messenger (Muhammad)
And those who believe and establish the prayer,
And give charity (alms) while they are (in Ruku`) bowing down. (al Máidah, 5:55)
When we say (`Alí is the Wali of Alláh) in our Adhán, it is based on this verse.