The Messenger grieving by the Statements of the Polytheists, and His Gratification
Allah says;
فلعلك تارك بعض ما يوحى إليك وضايق به صدرك أن يقولوا لوﻻ أنزل عليه كنز أو جاء معه ملك
So perchance you may give up a part of what is revealed unto you, and that your breast feels straitened for it because they say, "Why has not a treasure been sent down unto him, or an angel come with him!''
This statement of Allah, the Exalted, to His Messenger comforted the worries that the polytheists were causing him due to their statements directed towards him.
This is just as Allah says about them,
وقالوا ما لهذا الرسول يٲكل الطعام ويمشى فى اﻻسواق لوﻻ أنزل إليه ملك فيكون معه نذيرا
أو يلقى إليه كنز أو تكون له جنة يٲكل منها وقال الظلمون إن تتبعون إﻻ رجﻼ مسحورا
And they say: "Why does this Messenger eat food, and walk about in the markets. Why is not an angel sent down to him to be a warner with him!'' Or; "(why) has not a treasure been granted to him, or why has he not a garden whereof he may eat!'' And the wrongdoers say: "You follow none but a man bewitched.'' (25:7-8)
Thus, Allah commanded His Messenger and guided him to not let these statements of theirs grieve his heart. Allah directed him to not let these statements prevent him, or deter him from calling them to Allah, both day and night.
This is as Allah said,
ولقد نعلم أنك يضيق صدرك بما يقولون
Indeed, We know that your breast is straitened at what they say. (15:97)
Allah says in this verse,
فلعلك تارك بعض ما يوحى إليك وضايق به صدرك أن يقولوا
So perchance you may give up a part of what is revealed unto you, and that your breast feels straitened for it because they say...
The meaning here is that he (the Prophet) may be compelled to give up the Message due to what they (the polytheists) say about him.
However, Allah goes on to explain:
إنما أنت نذير
But you are only a warner.
"You (Muhammad) are only a warner and you have an example in your brothers of the Messengers who came before you. For verily, the previous Messengers were rejected and harmed, yet they were patient until the help of Allah came to them.''
والله على كل شيء وكيل
And Allah is a Guardian over all things.
An Explanation concerning the Miracle of the Qur'an
Then Allah says;
أم يقولون افتراه قل فٲتوا بعشر سور مثله مفتريات
وادعوا من استطعتم من دون الله إن كنتم صادقين