That the Messenger of Allah (ﷺ) said: "Whoever spends a sum in the cause of Allah, it is recorded for him seven-hundred fold."

There are something on this topic from Abu Hurairah.

This Hadith is Hasan, we only know of it from the narration of Ar-Rukain bin Ar-Rabi' (a narrator in the chain of this Hadith)
Jami` at-Tirmidhi Book 22, Hadith 7
← Previous Hadith That the Prophet (ﷺ) said, "Whoever fasts a day in the cause of Allah, Allah shall put between him and the Fire a trench whose distance is like that between the heavens and the earth."

This Hadith is Gharib as a narration of Abu Umamah.

Jami` at-Tirmidhi Book 22, Hadith 6 Next Hadith → That he asked the Messenger of Allah (ﷺ), "Which charity is most virtuous?" He said, "The service of a worshipper in the cause of Allah, or providing the shade of tent, or mount in the cause of Allah."

This Hadith has been reported from Mu'awiyah bin Salih in Mursal form.And Zaid has been contradicted concerning part of its chain.

He said: And Al-Walim bin Jamil has reported this Hadith from Al-Qasim Abu 'Abdur-Rahman, from Abu Umamah, from the Prophet (ﷺ).

Jami` at-Tirmidhi Book 22, Hadith 8