In one narration the wording is: "When Ibn Umm Maktum gives the call to prayer (adhan), keep eating and drinking"; that is, there is no doubt in this narration. (Allamah al-Haythami says:) I say:
Al-Nasa'i (may Allah have mercy on him) recorded it in abridged form.
Ahmad recorded it, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 4862