Megastar Amitabh Bachchan may have sung numerous songs for his films but the legendary actor surprised musical duo Vishal-Shekhar while recording a song for his soon to be released movie 'Aladin'.

The song 'O Re Saawariya' is already famous for Bachchan's signature pink pagadi and white kurta from his 'Namak Halaal' days, Vishal said.

The song is sung by Sudesh Bhonsle. While working on the song, at the spur of the moment, Bachchan came up with an alaap and hearing that, the music directors Vishal and Shekhar decided to incorporate the alaap in the song.

"The alaap at the end of the song was never planned. Mr Bachchan came up with this impromptu addition which turned out so well that we decided to make it a part of the song," Vishal said.

What impressed everyone all the more, was the fact that Bachchan, who is not really professionally trained, was able to sing it so well.

Doing an alaap is not easy and requires thorough practice. But Amitji did it wonderfully and thats really amazing, the music director added.

Interestingly, the films theatrical promo opens with this rendition in Big B's distinctive baritone.

Directed by Sujoy Ghosh, 'Aladin', also starring Sanjay Dutt, Ritesh Deshmukh, releases on October 30.