I heard the Messenger of Allah ﷺ say, "Seek out the weak among you, for you are only granted provision and victory through your weak ones."
أخبرنا يحيى بن عثمان قال حدثنا عمر بن عبد الواحد قال حدثنا ابن جابر قال حدثني زيد بن أرطاة الفزاري عن جبير بن نفير الحضرمي أنه سمع
سمعت رسول الله ﷺ يقول ابغوني الضعيف فإنكم إنما ترزقون وتنصرون بضعفائكم
Its isnad is sahih. Abu Dawud narrated it in the Book of Jihad, chapter on seeking victory through the surplus of horses and the weak, hadith 2594, from the hadith of Abdul-Rahman bin Yazid bin Jabir, with this wording. It is also in Kubra, hadith 4388. Tirmidhi said, in hadith 1702, that it is hasan sahih, and Ibn Hibban graded it sahih, hadith 1620, as did Hakim, 2/145.