It is narrated from Sayyiduna Abu al-Darda (may Allah be pleased with him) that he said: When the Prophet (peace and blessings be upon him) was fasting, he would order a man to stand on a raised part of the ground, and when he said: "The sun has set," he would break his fast.
Al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains al-Waqidi, who is da'if (weak), and he has been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 4879