It is narrated from Sayyiduna Mughira (may Allah be pleased with him) that when Sayyiduna Amir al-Sha'bi (may Allah be pleased with him) was asked about something, he would say «لا أدري» (I do not know). If he was asked again, he would say firmly: I swear by Allah for you, if I had knowledge of it (that is, by Allah, I do not have knowledge of it).
Hadith Referenceسنن دارمي / مقدمه / 188
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده ضعيف لضعف محمد بن حميد، [مكتبه الشامله نمبر: 188]
Hadith Takhrijاس روایت کی سند ضعیف ہے۔ خطیب نے [الفقيه والمتفقه 184/2] میں اس روایت کو ذکر کیا ہے، لیکن اس کی سند بھی ضعیف ہے۔