In one narration the words are: Sayyiduna Amr (may Allah be pleased with him) submitted: "I forbade them from lighting fires out of fear that the enemy would gauge how few they were."
Al-Tabarani recorded it with two chains; the narrators of the first are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجهاد / 9626