TT #307 · Parashas Reeh · Sixth Aliyah · Passuk 15:10
You shall surely give him, and your heart shall not be grieved when you give to him, because for this thing Hashem your God will bless you in all your work and in all that you put your hand to.
Torah Temimah points to Sifrei / Bava Metzia.
Sefaria — Sifrei / Bava Metzia
English: "Because of this thing (davar)" — from here we learn that if one said he would give charity and then gave, he receives reward for the saying and reward for the deed; likewise if he said he would give and was unable to give, or told others to give, or merely spoke kindly to the poor, he receives reward for that.
Derash on the verse (Sifrei) — reward for the verbal commitment to give.
One receives reward both for the promise to give charity and for the actual gift; even a kind word to the poor earns reward.