The companions asked, "Messenger of Allah, which jihad is best?"He replied, "That of one whose horse is hamstrung and whose blood is shed."
حدثنا وكيع حدثنا الأعمش عن أبي سفيان
قالوا يا رسول الله أي الجهاد أفضل قال من عقر جواده وأهريق دمه
Its chain of transmission is sahih. It is found in Darimi 2/264, in the book of jihad, on which jihad is best.