Then wait you for the Day when the sky will bring forth a visible smoke.
It was narrated that Masruq said,
"We entered the Masjid -- i.e., the Masjid of Kufah at the gates of Kindah -- and a man was reciting to his companions,
يَوْمَ تَأْتِي السَّمَاء بِدُخَانٍ مُّبِينٍ
(the Day when the sky will bring forth a visible smoke). He asked them;
`Do you know what that is?' That is the smoke that will come on the Day of Resurrection. It will take away the hearing and sight of the hypocrites, but for the believers it will be like having a cold."'
He said,
"We came to Ibn Mas`ud, may Allah be pleased with him, and told him about that. He was lying down, and he sat up with a start and said,
`Allah said to your Prophet
قُلْ مَأ أَسْـَلُكُمْ عَلَيْهِ مِنْ أَجْرٍ وَمَأ أَنَأ مِنَ الْمُتَكَلِّفِينَ
Say;"No wage do I ask of you for this, nor am I one of the pretenders." (38;86).
And it is part of knowledge that when a man does not know something, he should say, `Allah knows best.'
I will tell you a Hadith about that.
When the Quraysh did not respond to Islam and they grew stubborn, the Messenger of Allah invoked Allah against them that they would have years like the years (of drought and famine) of Yusuf. They became so exhausted and hungry that they ate bones and dead meat. They looked at the sky, but they saw nothing but smoke."'
According to another report;
"A man would look at the sky and he would see nothing between him and the sky except a smoky haze, because of his exhaustion."
فَارْتَقِبْ يَوْمَ تَأْتِي السَّمَاء بِدُخَانٍ مُّبِينٍ
يَغْشَى النَّاسَ هَذَا عَذَابٌ أَلِيمٌ