Narrated Salama bin Al-Akwa`: The Prophet (ﷺ) passed by some people of the tribe of Bani Aslam who were practicing archery. The Prophet said, "O Bani Isma`il ! Practice archery as your father Isma`il was a great archer. Keep on throwing arrows and I am with Bani so-and-so." So one of the parties ceased throwing. Allah's Apostle said, "Why do you not throw?" They replied, "How should we throw while you are with them (i.e. on their side)?" On that the Prophet (ﷺ) said, "Throw, and I am with all of you."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، حَدَّثَنَا حَاتِمُ بْنُ إِسْمَاعِيلَ، عَنْ يَزِيدَ بْنِ أَبِي عُبَيْدٍ، قَالَ سَمِعْتُ سَلَمَةَ بْنَ الأَكْوَعِ ـ رضى الله عنه ـ قَالَ مَرَّ النَّبِيُّ صلى الله عليه وسلم عَلَى نَفَرٍ مِنْ أَسْلَمَ يَنْتَضِلُونَ فَقَالَ النَّبِيُّ صلى الله عليه وسلم " ارْمُوا بَنِي إِسْمَاعِيلَ، فَإِنَّ أَبَاكُمْ كَانَ رَامِيًا ارْمُوا وَأَنَا مَعَ بَنِي فُلاَنٍ ". قَالَ فَأَمْسَكَ أَحَدُ الْفَرِيقَيْنِ بِأَيْدِيهِمْ. فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَا لَكُمْ لاَ تَرْمُونَ ". قَالُوا كَيْفَ نَرْمِي وَأَنْتَ مَعَهُمْ. قَالَ النَّبِيُّ صلى الله عليه وسلم " ارْمُوا فَأَنَا مَعَكُمْ كُلِّكُمْ ".
sunnah.com