Midrash Tanchuma, Lech Lecha 14

‎בַּמַּחֲזֶה לֵאמֹר אַל תִּירָא. זֶה שֶׁאָמַר הַכָּתוּב: אָז דִּבַּרְתָּ בְחָזוֹן לַחֲסִידֶיךָ וַתֹּאמֶר שִׁוִּיתִי עֵזֶר עַל גִּבּוֹר הֲרִימוֹתִי בָחוּר מֵעָם (תהלים פט, כ).

The Lord came unto Abraham in a vision, saying: “fear not” (Gen. 15:1). Scripture says elsewhere in allusion to this verse: Then Thou spokest in vision to Thy godly ones, and saidst: “I have laid help upon one that is mighty; I have exalted one chosen out of the people” (Ps. 89:20).

אָז דִּבַּרְתָּ בְחָזוֹן לַחֲסִידֶיךָ, זֶה אַבְרָהָם. שִׁוִּיתִי עֵזֶר, שֶׁעֲזָרוֹ הַקָּדוֹשׁ בָּרוּךְ הוּא עַל הַמְּלָכִים. הֲרִימוֹתִי בָחוּר מֵעָם, זֶה אַבְרָהָם, שֶׁנֶּאֱמַר: אַתָּה הוּא ה' הָאֱלֹהִים אֲשֶׁר בָּחַרְתָּ וְגוֹ' (נחמיה ט, ז). הֱוֵי, אַל תִּירָא אַבְרָם.

Then Thou spokest in vision to Thy godly ones refers to Abraham; I have laid help upon the one that is mighty alludes to the assistance which the Holy One gave him in his struggle against the kings; I have exalted one chosen out of the people refers to Abraham, as it is said: Thou art the Lord the God, who didst choose Abraham and send him forth (Neh. 9:7). Hence Scripture says; Fear not, Abraham.