UI Patterns (give ideas for mobile designs)

• Mobile-Patterns
• iospirations
• Pttrns
• Mobbin
• UI-patterns
• UI movement

UI Guidelines (could be good to get an idea of design conventions)

• iOS Human Interface Guidelines
• Google Material Design
• Microsoft Windows Design

Prototyping Design Toolkits (could be good to get an idea of design conventions and download ui components)

• iOS Toolkits
• Google Material Toolkit
• Microsoft Toolkits

UI Design Articles

• Summary of UI Design Do’s and Don’ts
• A Comprehensive Guide to Mobile App Design
• iOS vs Android App UI Design: The Complete Guide
• Button UX Design: Best Practices, Types, and States
• UI Design Glossary

Icon Design

• 10 Essential Tips for UI Icons
• Small Elements, Big Impact
• Designing for UI
• The Noun Project
• Free Icons Sets