lech lecha

The Parsha starts with לך לך מארצך ה' telling אברהם to go to the land that He's going to show him.  
When he gets there the Posuk says ויהי רעב בארץ that there was a hunger in the land & אברהם went down to Mitzrayim.
Rashi says that אברהם's Nisayon was that 'ה told him to come to this land where there's a hunger from which he's forced to leave & his Nisayon is not to second guess 'ה.
The Ohr Hachaim says that the reason 'ה gave him the Beracha of ואברכה מברכיך is because he followed 'ה even without knowing where he was going; he didn't question, he just followed 'ה.
So why does Rashi say that his Nisayon was to leave the land 'ה told him to go to? Maybe his Nisayon should have been to stay there & wait for 'ה to take care of him. The same way he went there with blind faith he should have stayed!
The Ramban takes it a step further & says that going down to Mitzraim was counted as an עון for אברהם for he should have known that 'ה would save him from death.
רב יחזקאל לוינשטיין.
offers an insight into why אברהם went down to Mitzrayim instead of staying.
Had אברהם stayed 'ה would have definitely taken care of him, but by doing that he would have been relying on an open miracle. If you rely on that & 'ה performs it for you, then you really have no doubt as to where your bread is coming from, so trusting in 'ה is easy.
But if you do things the normal way & you put in the effort & do what would be considered the right thing to do, & then things work out, it's easy for you to think that your  effort payed off & you did it yourself.
In that case it's much harder to say that it all came from 'ה!
That's what אברהם wanted - to strengthen his Emunah by doing things the normal way & then attributing it all to 'ה.
That's why rashi says that his Nisayon was to leave the land to go to where there is food, to act in the normal way & at the same time strengthening his Emuna by saying that everything came from ה!