It is narrated as marfu' from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), through Abu Hamzah from Muhammad (bin Ubaydullah). This Muhammad is al-Arzami, and he is matruk al-hadith (abandoned in hadith). It has also been narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) as connected (mawsul) through another weak chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشفعة / 11601