Awn said: I said to Umar bin Abd al-Aziz: Ibn Mas'ud (may Allah be pleased with him) used to say: "Soon there will be matters that are ambiguous. Whoever is absent from them but approves of them is like one who was present at them, and whoever is present at them but dislikes them is like one who was absent from them." And this greatly pleased him (Umar bin Abd al-Aziz).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8856