Musannaf Ibn Abi Shayba 21:345

Waki’ narrated to us, from Mis’ar, from ‘Abd al-Malik ibn Maysarah, from Muslim, from Suwayd ibn Jahbal, who said: Whoever says after the afternoon prayer (Asr), "There is no god but Allah, to Him belongs the praise, and He is capable of all things," it will fight on behalf of the one who said it until the same time the next day.