Narrated Salama: Allah's Messenger (ﷺ) passed by some people from the tribe of Aslam practicing archery. He said, "O children of Ishmael! Throw (arrows), for your father was an archer. I am on the side of Bani so-andso," meaning one of the two teams. The other team stopped throwing, whereupon the Prophet (ﷺ) said, "What has happened to them?" They replied, "How shall we throw while you are with Bani so-andso?" He said, "Throw for I am with all of you."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ يَزِيدَ بْنِ أَبِي عُبَيْدٍ، حَدَّثَنَا سَلَمَةُ ـ رضى الله عنه ـ قَالَ خَرَجَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَلَى قَوْمٍ مِنْ أَسْلَمَ، يَتَنَاضَلُونَ بِالسُّوقِ، فَقَالَ " ارْمُوا بَنِي إِسْمَاعِيلَ، فَإِنَّ أَبَاكُمْ كَانَ رَامِيًا، وَأَنَا مَعَ بَنِي فُلاَنٍ ". لأَحَدِ الْفَرِيقَيْنِ، فَأَمْسَكُوا بِأَيْدِيهِمْ فَقَالَ " مَا لَهُمْ ". قَالُوا وَكَيْفَ نَرْمِي وَأَنْتَ مَعَ بَنِي فُلاَنٍ. قَالَ " ارْمُوا وَأَنَا مَعَكُمْ كُلِّكُمْ ".
sunnah.com