Musnad Ahmad 1249:0

Narrated to us by Abd al-Rahman, from Sufyan, from Alqamah bin Marthad, from al-Yusr bin Safwan, from Safwan bin Abd Allah bin Safwan, from Busrah bint Safwan, that the Messenger of Allah (ﷺ) said: "Whoever touches his private part, let him perform ablution."