It is narrated from Bakr bin Abdullah al-Muzani (may Allah have mercy on him): I saw Ibn Umar (may Allah be pleased with them both) squeeze a pimple of his, and some blood came out of it, which he rubbed away between his two fingers; then he prayed and did not perform ablution.
A report with this meaning has been narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him), then from Sa'id bin al-Musayyab, Salim bin Abdullah, al-Hasan and Tawus (may Allah have mercy on them), and it is narrated from al-Qasim bin Muhammad (may Allah have mercy on him): There is no ablution for one who has been cupped.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 667